Dovetail attachment techniques between turbine buckets and turbine rotor wheels are well known in the art. It has been found, however, that conventional tangential entry dovetails on the latter stages of low pressure rotors operate in an environment that is conducive to stress corrosion cracking (SCC). SCC is accelerated by the stress levels that are present in the hook fillet region of typical dovetail configurations. Normally, these stresses are acceptable, but in contaminated steam, cracks can initiate and, if left undetected, grow to a depth that may cause failure of the wheel hooks. In extreme cases, all of the hooks may fail and buckets may fly loose from the rotor.
It has been found generally that the cracking problem described above occurs primarily in wheel hooks rather than in the complementary bucket hooks. This is apparently because the steels, such as NiCrMoV, used for low pressure rotors are much less resistant to SCC than are the 12 Cr steels used for buckets. The low pressure steels, however, give the optimum combination of properties available for overall low pressure rotor design considerations. Therefore, an effective means of reducing the probability of initiating SCC in the typical low pressure steam environment is not to change materials but, rather, to reduce the stresses in the wheel dovetail to acceptable levels. If the maximum stress in components operating in a corrosive environment is reduced below the yield strength of the material, the resistance to SCC is greatly improved.